1. Field of the Invention
This invention relates generally to semiconductor devices including flexible structures and, more particularly, to semiconductor sensors including capacitors, piezoresistors, and transistors with flexible structures partly supported by a pedestal.
2. Description of Related Art
Known micro-machined sensors are semiconductor devices with flexible structures that move or deform to change properties such as the capacitance, resistance, or transconductance of the devices. The changed property can be measured to determine the magnitude of the force that deformed the flexible structure. For example, a typical surface micro-machined capacitive sensor has a flexible plate (usually circular), suspended over a fixed substrate containing a conductive region. In this arrangement, the plate and the substrate form the two electrodes of a capacitor. A force on the flexible plate deforms or moves the flexible plate and changes the capacitance between the electrodes. A wide variety of capacitive sensors including pressure sensors and accelerometers use this principle. An important part of designing a good capacitive sensor is to maximize the change in capacitance with applied force because the change in capacitance is often the primary factor determining detector resolution.
Other types of semiconductor sensors include piezoresistive and transconductive devices. Semiconductor piezoresistive devices typically include a piezoresistive element formed in flexible diaphragm suspended over a cavity in a substrate. Deformation of the flexible diaphragm caused by an applied force changes the resistance of the piezoresistive element and allows measurement of the applied force. Moving gate field effect transistors have a flexible diaphragm that forms the gate of the transistor and is suspended over a cavity in a substrate containing source, drain, and channel regions of the transistor. An applied force deforms the gate and changes the threshold voltage of the transistor.
Surface micro-machining is a fabrication technique that allows the production of planar mechanical and electrical elements on a semiconductor wafer. The mechanical elements such as flexible diaphragms are most commonly formed using a polysilicon layer deposited on a sacrificial layer that is subsequently removed to provide gaps or cavities between the polysilicon layer and the underlying layer. A newer form of surface micro-machining employs a monocrystalline layer that is fusion bonded to a structured substrate. An important advantage of the newer form of surface micro-machining is that monocrystalline silicon possesses superior mechanical properties compared to polysilicon. In both cases, the need for subsequent photolithography and processing using standard semiconductor fabrication techniques limits the thickness of the mechanical layer, either the polysilicon layer or the monocrystalline silicon layer, to a few microns, typically less than 10 microns.
The combined requirements of maximizing sensor capacitance and limiting the thickness of the flexible structures lead to significant design trade-offs. For example, in a capacitive pressure sensor with a particular pressure range, the plate thickness limits the maximum diameter of a flexible plate and the minimum allowable nominal gap between the plates. Specifically, the thickness and span of the flexible plate must be such that the maximum measurable pressure and electrostatic attractive forces in the sensor do not cause the flexible plate to contact the fixed plate. These constraints limit the maximum attainable capacitance for the sensor. Equation 1 shows the relationship between the maximum pressure Pmax and the maximum plate deflection Wmax (at the center of a circular plate), EQU Wmax=Pmax R.sup.4 /64 D Equation 1:
In Equation 1, D is the flexural modulus of the flexible plate (for a given thickness and material), and R is the radius of the circular plate. There is no design flexibility in capacitive area as radius R is fully specified for a given Pmax, D, and Wmax. As an example, a typical 600 kPaA (kilopascals absolute) sensor has a nominal capacitance of 0.5 pF (picofarads). In order to increase the maximum capacitance, the designer must have control over an additional design parameter.
Hence, a structure is needed that can be manufactured by a process that provides design control over the area of the capacitive plates irrespective of pressure range and plate thickness.